1. Field of the Invention
The present invention relates generally to the provision of constant power to telephone subscriber loops and more particularly to circuitry for providing power feed through a small power converter having an isolated output circuit and a constant power characteristic.
The line circuit of the present invention may be used as a line interface circuit in a telephone central office switch, in a subscriber carrier system or in a channel bank. Other fields of application are as a subscriber line interface, a two-wire exchange line interface with loop start or ground start or as an originating two-wire trunk line interface.
2. Description of the Prior Art
The subscriber line feed function associated with a classic telephone circuit is essentially equivalent to two resistors which are designated as line feed resistors in series with the central office battery. The value of the battery is in the range of 48 to 60 volts, and the resistors typically vary between 200 to 800 ohms depending upon the particular telephone exchange. The line feed resistors are selected at the central office to be of a value necessary to produce the proper line current versus loop resistance characteristics as expected by the telephone instrument. As is understood, a long telephone line indicates a large resistance, while a short telephone line has less of a resistance. The line feed resistors are placed in series with the telephone line and are selected according to the desired loop current characteristics. In any event, these resistors dissipate a relatively large amount of power and cause thermal problems at the central office.
Various telephone subscriber line interface circuit techniques are known in the prior art for controlling telephone subscriber loop current. Examples of circuitry for implementing such prior art techniques are provided with reference to U.S. Pat. Nos. 4,161,633, Subscriber Line/Trunk Circuit; 4,315,106, Apparatus for Regulating Current Supplied to a Telephone Line Circuit of the Type Employed in Digital Telephone Systems; 4,317,963, Subscriber Line Interface Circuit Utilizing Impedance Synthesizer and Shared Voltage Source for Loop Current Regulation Control; and 4,387,273, Subscriber Line Interface Circuit with Impedance Synthesizer, all of which describe techniques for the provision of regulated line current to telephone subscriber loops, and all of which are assigned to the same assignee as the present invention.
Other line feed circuits of the prior art have consisted of a transformer with a split winding on the telephone side, a pair of matched resistors connected from the inside of the split winding to battery and ground, with the outside of the split winding being connected to the tip and ring wires of the subscriber loop that are coupled to the subscriber's telephone; and a third winding connected to the voice frequency (VF) circuits in the telephone line interface circuitry. An example of another circuit of the prior art can be found with reference to the Bell System Technical Journal, Vol. 61, No. 4, April 1982, pp. 451-455, titled Pheripheral Systems Architecture and Circuit Design, J. M. Adrian, et al.
FIG. 1 is an example of a prior art telephone line circuit. With reference to the line circuit of FIG. 1, the transformer 10 must pass the current that operates a telephone coupled to tip and ring lines 12 and 14. Thus, transformer 10 must either be a large transformer with a gapped core, or a smaller transformer with a fourth winding and an additional circuit to generate an opposing dc current to cancel the flux in the core. Such a circuit has a number of drawbacks.
When the external loop is short, i.e. low resistance, a relatively high current flows in the circuit. Since the battery voltage is fixed, most of the power supplied is dissipated in the feed resistors and transformer windings. This results in an unnecessarily high power required to drive the telephone circuit and a high dissipation in the central office line interface equipment.
Surges from lightning or ac power crosses go to battery and ground through the transformer windings and feed resistors. These are relatively low resistances, so large currents flow. It is difficult to protect the line circuits from these large currents, hence large components are required.
A short circuit from the side of the line connected to battery and to ground has only one feed resistor and a transformer winding to limit current. A large current will flow and large feed resistors and transformer wire size are required to withstand this current.
Other line circuits of the prior art do not provide a constant power feed but rather provide lower power output as the subscriber loop resistance is decreased. Such techniques of the prior art may result in lower power dissipation; however, the current output under short loop, i.e. low resistance conditions, would be too low to fully activate the varsitors in the telephone to reduce voice frequency levels. This results in the telephone sounding too loud unless additional circuit means are provided at the central office switch to detect short loops and attentuate the voice frequency level appropriately.